Volcanoes Spawn Mini-Cyclones, Then Lightning "Sheaths"

Ker Than
for National Geographic News

March 25, 2009

A newfound link between two of nature's most violent phenomena could explain how volcanic ash clouds can generate bolts of lightning and tornado-like dust devils and waterspouts.

Scientists have long known that tornadoes are the products of colossal columns of spinning air—mesocyclones—inside large storm clouds.

A new study suggests mesocyclones can also form in the ash plumes of volcanic eruptions.

Under certain circumstances, these volcanic mesocyclones can aid in lightning production and create tornado-like structures that corkscrew toward the Earth, said study team member Pinaki Chakraborty of the University of Illinois at Urbana-Champaign.

Chakraborty suspects the recent eruptions of Redoubt Volcano in Alaska "might be powerful enough" to generate mesocyclones.

(See photos of Redoubt Volcano erupting.)

So far, however, no lightning or cyclones have been observed at Redoubt Volcano, said Michelle Coombs, a geologist at the Alaska Volcano Observatory, who did not participate in the study.

Volcanic Cyclones' Model Behavior

While examining satellite images of the 1991 eruption of Mount Pinatubo in southeast Asia, the study team discovered that the volcanic plume was rotating around its axis in a way that couldn't be explained by conventional theories.

"This was not known before," Chakraborty said.

Using results from thunderstorm computer modeling, the team showed how powerful updrafts generated during a volcanic eruption can cause coil-like tubes of circulating air to flip their orientations from horizontal to vertical within the plume.

The resulting volcanic mesocyclone could cause the entire plume to rotate like a tornado.

In theory, the spinning could push charged ash and dust particles outward, which would explain the enveloping "sheath" of lightning seen around some volcanic plumes.

(See a picture of a spectacular instance of a volcanic lightning sheath.)

Volcanic mesocyclones, the model suggests, could also be responsible for downward-pointing spinning funnels of air which can suck up dust or water to create dust devils or waterspouts, respectively.

When the same process occurs in thunderclouds, the result is a tornado, Chakraborty explained.

Ken Wohletz is a volcanologist at Los Alamos National Laboratory in New Mexico who was not involved in the study.

Wohletz said the team is "likely on the right track" with their theory.

"What is really neat about their model is that it provides the basic framework that helps explain the attendant meteorological phenomena" such as lightning and tornado-like cyclones, Wohletz said.

Findings detailed in the March 26 issue of the journal Nature.